Service outages can occur in systems which broadcast data, video, audio and other information using radio frequencies. These outages can prevent receivers, and particularly mobile receivers, from receiving the broadcast service altogether, or cause them to receive a signal so degraded that the service is rendered unacceptable. These outages are generally due to physical blockage of transmission paths between the transmitter and receiver (e.g., due to mountainous terrain or long tunnels) and multipath fading and reflection of the transmission path.
Satellite broadcast systems can use two transmission channels to provide time and/or space diversity for mitigating service outages due to multipath, physical blockages and interference in mobile broadcast receivers. These time diversity systems, however, are disadvantageous for reasons which will be illustrated below in connection with FIG. 4. FIG. 1 depicts a satellite broadcast system 10 employing time diversity which comprises at least one geostationary satellite 12 for line of sight (LOS) satellite signal reception at receivers indicated generally at 14. Another geostationary satellite 16 at a different orbital position is provided for time and/or space diversity purposes. The system 10 further comprises at least one terrestrial repeater 18 for retransmission of satellite signals in geographic areas where LOS reception is obscured by tall buildings, hills and other obstructions. The receivers 14 can be configured for dual-mode operation to receive both satellite signals and terrestrial signals and to combine or select one or both of the signals as the receiver output. However, it will be understood that, where the receivers are in a fixed location, it is sufficient for such receivers to operate by receiving signals from a single source and that is may reduce the cost and complexity of such receivers if they are designed for single mode operation.
The satellite broadcast segment preferably includes the encoding of a broadcast channel into a time division multiplexed (TDM) bit stream. The TDM bit stream is modulated prior to transmission via a satellite uplink antenna. The terrestrial repeater segment comprises a satellite downlink antenna and a receiver/demodulator to obtain a baseband TDM bitstream. The digital baseband signal is applied to a terrestrial waveform modulator, and is then frequency translated to a carrier frequency and amplified prior to transmission.
The problem associated with broadcast systems based on time diversity can be understood from FIGS. 2-4. With reference to FIG. 2, a transmission channel 60 from a late satellite, for example, is delayed by a predetermined amount of time (e.g., ten 432 millisecond (ms) frames) with respect to the other channel 62. Receivers are therefore configured to receive both transmission channels 60 and 62 and to add an identical delay to the channel 62 that was not earlier subjected to the predetermined amount of delay. With reference to FIG. 3, the two received streams 64 and 66 are then compared and combined as indicated at 68. In optimal situations, the combined stream 68 is a continuous stream of the original broadcast, even though one or both of the channels 60 or 62 may not have been receivable during a temporary service outage. This is true if the data transmitted during the outage was successfully received from the other channel during the outage period or, in cases where both channels are blocked simultaneously, if the outage does not exceed the time delay between the channels. As an illustration of the latter situation, the signal blockage 70 that occurred in both of the two recovered bit streams 64 and 66 of FIG. 3 (i.e., the loss of frames 10 through 19 in channel 60 and loss of frames 20 through 29 in channel 62) is recovered in the combined recovered bit stream 68. With reference to FIG. 4, problems in recovering the source data stream for channels 60 or 62 can occur when one of the satellite paths is completely blocked due to terrain, for example. The blocked signal 72 (i.e., frames 23 through 27) in the early satellite channel cannot be recovered from the late satellite channel, resulting in an audio mute interval 74, as shown in the recovered data bit stream 68. This audio mute interval 74 is an error interval that is too large to be mitigated by error concealment techniques. As stated previously, satellite broadcast systems can be reinforced using terrestrial repeaters. While a repeater can be used to provide for the transmission of the source data stream when LOS signal reception of a satellite channel is obstructed, repeaters represent a substantial additional system cost and are generally only implemented in urban centers and suburban areas. Accordingly, a need exists for a satellite broadcast system which provides error concealment in a single satellite coverage environment without requiring a terrestrial reinforcement system.
Another approach for minimizing the effect of noise bursts and fading in a data transmission system involves spreading source bits over time in a data stream using interleaving. An interleaver is generally implemented using a block structure or a convolutional structure.
Using a block structure, a matrix of predetermined size is selected (e.g., m rows and n columns). An input data stream is read into a shift register matrix. The bits in the data stream fill consecutive matrix rows with data folding into the next row as each row is filled. The separation of data elements in a column is therefore n bits, which corresponds to the interleaving depth being used. The data elements in each column are then coded and transmitted by row. The received bits are applied to an identical shift register matrix at the decoder. Data elements are decoded per column prior to being read out per row. When a noise burst occurs to all bits in a single row of an interleaved word (i.e., for n*c seconds wherein c is the bit period), only one bit of the coded word is corrupted. The n bits of the affected row can be corrected individually.
Unlike a block interleaver, which interleaves blocks of data independently of each other, a convolutional interleaver is a feed-forward type of coder which continuously produces an output. A block interleaver, on the other hand, assembles and stores blocks of bits prior to interleaving. Block interleavers have disadvantages. A block interleaver cannot fully decode a received data stream until all of the m*n bits, as set forth in the previous example, arrive at the receiver and are de-interleaved. The size of the matrix therefore is an important consideration. A need therefore exists for an interleaving method which operates on a continuous data stream, which allows for relatively simple de-interleaving at the decoder, and which is not subject to the problems associated with block interleaving.